Kepler: Opening the Planet Verification Bottleneck

byPaul GilsteronFebruary 27, 2014

A planet like Kepler-296f is bound to get a lot of publicity. Orbiting a star half the Sun’s size and only five percent as bright, this world, twice the size of the Earth, appears to orbit in the habitable zone, where liquid water could exist on its surface. We focus so much on the potential of life that the four planets announced yesterday (out of 715 newly verified worlds) inevitably get special treatment. And we learn that Kepler-296f exists in a system with four other planets, orbiting the star every thirty days. What we don’t know is whether we’re dealing with a small Neptune-class world surrounded by a thick hydrogen/helium atmosphere or a water world with a deep ocean.

An interesting world, to be sure, but the real story in yesterday’s announcements from the Kepler team has to do with the ‘verification by multiplicity’ technique used to validate the existence of so many planets in 305 star systems. One of the findings papers titled “Almost All of Kepler’s Multiple Planet Candidates are Planets” (to be published March 10 in The Astrophysical Journal) makes the case that the vast majority of Kepler’s multiple planet candidates are true multi-planet systems, the number of false positives lower for multiples than for single detections. The paper continues (internal citations removed for brevity):

Kepler has found far more multiple candidate systems than would be the case if candidates were randomly distributed among target stars. False positives are expected to be nearly randomly distributed among Kepler targets, whereas true transiting planets could be clustered if planets whose sizes and periods are adequate for transits to be detected often come in multiples, as is the case for planets detected by radial velocity variations, and/or if planetary systems tend to be flat, so geometry leads to higher transit probabilities for other planets if one planet is seen to transit.

The paper gives a blow by blow description of the statistical analysis used here, but for our purposes, Kepler looked at over 150,000 stars and found only a few thousand that showed the characteristic lightcurve of a planetary transit. If this transit-like pattern were random, only a few stars would have shown more than one pattern, but we find hundreds of stars that have multiple transit-like patterns. As Jack Lissauer (NASA Ames) pointed out in yesterday’s teleconference, multiplicity is not random. Instead, it is a technique for opening up what Lissauer calls the ‘planetary verification bottleneck.’ Joining Lissauer in the teleconference were Jason Rowe (SETI Institute), Douglas Hudgins (NASA Astrophysics Division), and Sara Seager (MIT).

Image: The histogram shows the number of planets by size for all known exoplanets. The blue bars on the histogram represents all the exoplanets known, by size, before the Kepler Planet Bonanza announcement on Feb. 26, 2014. The gold bars on the histogram represent Kepler’s newly-verified planets. Image Credit: NASA Ames/W Stenzel.

Lissauer talks about the method as a way ‘to verify multiple planet candidates in bulk to deliver planets wholesale,’ and it yields 715 worlds of which 94 percent are smaller than Neptune, in the process establishing that multiple planet systems around a single star are common. The number of confirmed exoplanets now nears 1700. Besides Kepler-296f, three other verified worlds are less than 2.5 times the size of Earth and orbiting in their star’s habitable zone.

Overall, these results offer the largest number of validated planets ever to be announced at one time, a number sufficient for the transit technique to overtake radial velocity as the most prolific technique for exoplanet detection. And as Jason Rowe pointed out, these results show us small planets in multiple planet systems that move in circular, flat orbits, a configuration not unlike what we find in our own inner Solar System. “The more we explore,” Rowe added, “the more we find familiar traces of ourselves amongst the stars that remind us of home.”

The paper cited above is Lissauer et al. “Almost All of Kepler’s Multiple Planet Candidates Are Planets,” in press at The Astrophysical Journal (preprint). See also Rowe et al. “Validation of Kepler’s Multiple Planet Candidates. III: Light Curve Analysis & Announcement of Hundreds of New Multi-planet Systems,” also in press at The Astrophysical Journal (preprint). The archived Kepler media teleconference can be accessed here.

Comments on this entry are closed.

Harry R RayFebruary 27, 2014, 12:10

I’m overjoyed at this news for two specific reasons! ONE: One of the systems (Kepler 186) has a KOI designated number of 571. On the HEC website, KOI571.05 is only one of two “warm terrans” listed! Alas, only 0.1 through 0.4 were confirmed, but that is because only the first two years of data were used in the study, and 0.5 is a relative newcomer. Special attention should be given to this system to see whether 0.1-0.4 cause TTV’s in 0.5 that would verify IT as a planet,too! Now, what REALLY overjoys me is that one of these systems consists of two stars, one with 2 planets orbiting it, and,one planet orbiting the other star! CORRECT ME IF I AM WRONG< but i believe this the first binary system in which BOTH STARS have at least one planet! I do not know either the KOI or the Kepler designation for this system, and I would appreciate it if someone helps me find it.

Nice finding, most important is the miniscule number of Terrestrial planet
candidates in the HZ of stars. Four, that’s the truth of it, and we can call them terrestrial only technically.

Has the Kepler team publicly acknowledged that the spacecraft is unable
to detect a world similar to our own orbiting a sun somewhat similar to ours.
to the PRESS. Or is that info in the footnotes in press releases. I know it is not anybody’s fault how things turned out on the sensitivity question. But having to “interpolate” that earth sized worlds do exists in the HZ of other stars is maddening frankly. I am sure others can find more eloquent expressions of disappointment.

The other paper that came out with this announcement is this one by Lissauer et al.. It notes that Kepler-296 = KOI-1422 is a binary system, and it does not seem to be clear which of the two stars the planets orbit. If orbiting the secondary, Kepler-296f ends up with a radius over 3 times that of the Earth, also the insolation would be different. Even around the primary the inferred radius is roughly twice that of the Earth, and there are good reasons to be sceptical that planets of this size are anything other than mini-Neptunes.

From what I can see, this object should not be promoted as a habitable planet candidate. NASA should really do better than this, exoplanets are an interesting field but the hyping up of (at best) marginal candidates for habitability is probably going to end up being rather damaging. It would be a pity if this causes habitable planets to no longer be seen as newsworthy by the time a genuinely good candidate is found.

These multiplicity findings are from only the first 2 years of Keplers’ observations. Finding Earth size planets in the longer period HZ orbits are at the very limits of Keplers’ capabilities. It will be very interesting to see the results from a full analysis of all 4 years of observations. I don’t think we will have to wait very long for more announcements.

I think Earth size planets in HZ orbits around FGK and M main sequence dwarfs may exist in abundance even if not in majority. Why would they not?
But it will take more observations and time to give us a better idea of their populations. One must beware of observational bias. A few years ago it seemed as if hot Jovians were the most common planets. Better instruments and better observations will ofcourse give us a clearer idea of planetary characteristics.

The Kepler team have done a wonderful job and deserve a hearty congratulations for their ingenuity, dedication and resourcefulness.

It seems that, while many of these planets are in the habitable zone, they will likely be tidal locked since they are too close to the star. Unfortunately, Kepler couldn’t keep looking for enough time to find planets with similar conditions to Earth. That’s sad.

Probably these planets the size of Earth, in case they have a meaningful atmosphere, will be plagued by extremely fast winds in the temperate zones, or be too hot near the equator during the day and too cold during the night. The zones near the pole might be the only places to life like earth to develop. But that’s a too small zone to observe.

Going from candidates to verified planets for this many worlds will go down as an important result in the history of exoplanet science. Strength of results, high confidence, and stringent vetting are the hallmarks of good science.

Question: does anyone happen to know what is the most up to date figure for the false positive percentage for all of Kepler’s candidates– not just the ones in multiple systems addressed in this latest study? A cited figure from the literature would be greatly appreciated. Thanks!

Some essential conclusions from the two cited papers plus the related one cited by andy:

– More than 99% (probably 99.8%) of the multi-planet candidates are indeed planets, and at least over 90% of single-planet candidates. My own take: this implies that we might as well use all the planet candidates of the KOI catalogue for statistics, instead of just the confirmed ones.

– Very hot planets (with periods of a few days) occur overwhelmingly in single-planet systems. We knew that already for hot Jupiters, but this seems to be true for all very close orbit planets.

– More worrisome: “There is also a small deficit of multi-planet candidates relative to singles at periods above 100 days.” However, this is probably a result of observational bias (difficulty of observation of smaller planets in wider orbits).

Also, with ref. to RobFlores and Rocha, and in agreement with Mike: the shortage of small planets in HZ orbits is still largely due to observational bias: only the first 2 years of Kepler data have been analyzed this way. It remains to be seens if, as suggested by some studies, there is a real sharp drop-off of smaller planets at wider orbits, or not.

What strikes me about the bar chart above is particularly that the number of Jovian gas giants hardly increases. Is this due to the verification method? I mean, that the Jovians were already well-known previously and are hardly added this way?
Also striking and somewhat puzzling to me is the fact that the Jovian and larger category is still the largest, although studies have indicated that the medium-sized (super-earth/Neptune) classes should be the largest. And that it contains about confirmed 700 planets, nearly all of them previously know.
However, Kepler data realease of late 2013 of all planet candidates shows only about 330 candidates of these sizes. How can this be reconciled?

See for instance: http://upload.wikimedia.org/wikipedia/commons/c/cc/Size_of_Kepler_Planet_Candidates.jpg
If indeed by far most of the candidates are planets, then we may take consider this graph as reliable and representative, apart from the mentioned mystery of the recent over-abundance of Jovians (still absent in this graph).
This Kepler graph clearly shows the mentioned abundance of medium-sized planet sizes.
With time proceeding and the abundance of earthsized planets ( < 1.25 Re), I would think it is only a matter of time before earthlike planets start showing up in abundance in the HZ of solar type stars.

“Origin and loss of nebula-captured hydrogen envelopes from ‘sub’- to ‘super-Earths’ in the habitable zone of Sun-like stars” by lammer et al.

This study makes is convincingly clear that the vast majority of planets greater than about 2 Me, which corresponds to about 1.25 Re, are not true terrestrials, but rather mini-Neptunes, with a very large and dense H/He atmosphere. The paper’s conclusion even mentions 1.15 Re, but the body of the text is a bit more tolerant (I think 1.2 Re or so). Above this the heavy H/He envelope will not erode sufficiently even close to a star and over the entire lifespan of the (solar type) star. As a lower limit for terrestrial planets with an a sustainable atmosphere 0.8 Re is mentioned.

Note that ´Kepler´ also mentions 1.25 Re as an upper limit for earthlike.

This implies that, unless very convincing information comes in proving otherwise, as for now we should not consider any Kepler planet outside the 0.7 – 1.3 Re range as earthlike.

Ronald:
“What strikes me about the bar chart above is particularly that the number of Jovian gas giants hardly increases. Is this due to the verification method? I mean, that the Jovians were already well-known previously and are hardly added this way?
Also striking and somewhat puzzling to me is the fact that the Jovian and larger category is still the largest, although studies have indicated that the medium-sized (super-earth/Neptune) classes should be the largest. And that it contains about confirmed 700 planets, nearly all of them previously know.
However, Kepler data realease of late 2013 of all planet candidates shows only about 330 candidates of these sizes. How can this be reconciled?”

That’s because the histogram shows ALL exoplanets ever discovered, not just Kepler’s discoveries.
I suppose most Jovian candidates are in single-planet systems due to their orbital dominance, so they can’t be verified by this method.

Holger: thanks, that makes a lot of sense.
However, at the same time this makes the chart somewhat less interesting to me, because it suggests an accuracy that there isn’t. I mean that it is comparing apples and oranges.
Then the chart of all Kepler planet candidates, refered to by me above, may be more telling. I am eagerly awaiting the analyzed results of all Kepler data up to its breakdown.

@ljk: “Perhaps we should stop assuming that all life comes from – or ends up moving to – Earthlike planets. Again we assume the Universe was made just for us and beings like us”.
I usually agree with you but just now I differ: we may assume that the entire universe follows the same fundamental rules. Therefore it is reasonably safe also to assume that biological life also follows more or less the same basic rules (water, carbon, probably oxygen for higher life, etc.). It can hardly be a coincidence that, on a very silicon dominated planet, we are all carbon-based life.
And if super-earths are really mini-Neptunes, with H-rich atmospheres, that may drastically change the conditions for that life, both for starting it, and maintaining it.
BTW, it is for the same reason, that I strongly believe that we should seek life in the first place on planets around solar type stars, and not M-dwarfs: they are by far the most common, and yet our own star is is a G-star. A coincidence, or significant?

Main results:
– “M dwarfs are hosts to an abundance of low-mass planets and the occurrence rate of planets less massive than 10 Me is of the order of one planet per star, possibly even greater”;
– “planets with masses between 3 and 10 Me are common in the stellar habitable zones of M dwarfs with an estimated occurrence rate of 0.2 planets per star”.

BTW, the first result is in accordance with an earlier study.
In view of recent study with regard to habitable planet size, I would not call 3 – 10 Me habitable, but maybe there are smaller ones as well and 3 Me was the detection limit.

When I look for habitable zone, I think I am talking about an orbital zone where a planet metazoan, like we find on Earth, can survive. If I was merely talking about life, and given that we know that life on Earth can survive at extreme range, I may end up with enormous an arbitrary range of conditions for life. And we just know life on Earth.

It would be alright if we could send a satellite to look closely for such planets with “weird” life. But it won’t happen so soon. So, if I want to talk about an “habitatle” zone, I should restrict myself with what I compare to Earth, since this is the only model we have to compare with.

Finding a planet that is too close to a start that it is tidal locked to a star will hardly harbor life that is similar to Earth, as far as I can see. Kepler has only been able to search for planets in too small stars, that are out of the habitable zone, as I tried to define. The bad news is that Kepler won’t anymore be able to find a planet, or at least it will be very unlikely, to find such planet.

The worst news it is that the next telescopes, that look for planets, will not be able since they won’t be looking for an “Earth prime”.

Paul and others: I just completed my own analysis of *all* Kepler planet candidates found up to now (3845), and looked in particular at earthlike planets (R between 0.8 and 1.25 Re) and planets in the HZ of their star. I also related the different Kepler size categories to orbital (AU) distance, which we normally don’t see in the press releases. The results are rather interesting and I could share them here or somewhere else (it is quite a bit), if people are interested.

Scientists are close to announcing the first Earth-sized planet in a habitable zone around its parent star.

Astronomer Thomas Barclay, with NASA’s Ames Research Center in California, culled data collected by the Kepler space telescope to ferret out a five-planet system, the outermost of which circles toward the outer edge of its star’s habitable zone, according to reports posted Wednesday on Twitter by astronomers attending the Search for Life Beyond the Solar System conference in Tucson, Ariz.

NASA astronomer finds first Earth-sized planet in habitable zone
The search for a new Earth outside the solar system seems to be nearing its end. NASA’s Ames Research Center astronomer Thomas Barclay has found a planet nearly the size of Earth in the habitable zone of a star in the Milky Way.

Barclay’s announcement at the Search for Life Beyond the Solar System conference hasn’t been officially published yet, so the details are scarce. We know that:

1. It’s an M1 red dwarf star (maybe we should call it Krypton.)

2. It’s a goldilocks planet, orbiting within the zone where liquid water (and life) can exist.

3. It’s radius is only 1.1 times the size of Earth. Until now the minimum size for a new Earth candidate was 1.4 times – Kepler-62f, which orbits a star about 1,200 light years away from us.

@LJK – If we delve in details, Krypton was probably some kind of an exotic heavy matter “Super-Earth”, that made the inhabitants so strong and accustomed to super-high gravity that one of them (Superman) when visiting Earth actually could “fly”/”jump” to orbital distances.

As far as the article and news are concerned. I agree with Mike. Certain people are jumping the gun with conclusions. Our current technology for detection has been very limiting. I say we give the benefit of the doubt to future missions carrying equipment with finer precision capabilities. When we have the tech to detect Pluto-sized objects around any type of star and any distance from it, yet the findings show a common trend of mini-neptunes and gaseous super-earths, then I will join you guys on the “Rare Earth” bandwagon.

Until then, all such conclusions are based on speculation and limited data, nothing else.

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last eleven years, this site has coordinated its efforts with the Tau Zero Foundation, and now serves as the Foundation's news forum. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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